Conventional ultrasound imaging systems comprise an array of ultrasonic transducers which are used to transmit an ultrasound beam and then receive the reflected beam from the object being studied. For ultrasound imaging, the array typically has a multiplicity of transducers arranged in a line and driven with separate voltages. By selecting the time delay (or phase) and amplitude of the applied voltages, the individual transducers can be controlled to produce ultrasonic waves which combine to form a net ultrasonic wave that travels along a preferred vector direction and is focused at a selected point along the beam. Multiple firings may be used to acquire data representing the same anatomical information. The beamforming parameters of each of the firings may be varied to provide a change in maximum focus or otherwise change the content of the received data for each firing, e.g., by transmitting successive beams along the same scan line with the focal point of each beam being shifted relative to the focal point of the previous beam. By changing the time delay and amplitude of the applied voltages, the beam with its focal point can be moved in a plane to scan the object.
The same principles apply when the transducer is employed to receive the reflected sound (receiver mode). The voltages produced at the receiving transducers are summed so that the net signal is indicative of the ultrasound reflected from a single focal point in the object. As with the transmission mode, this focused reception of the ultrasonic energy is achieved by imparting separate time delay (and/or phase shifts) and gains to the signal from each receiving transducer.
Such scanning comprises a series of measurements in which the steered ultrasonic wave is transmitted, and the reflected ultrasonic wave is received and stored. Typically, transmission and reception are steered in the same direction during each measurement to acquire data from a series of points along an acoustic beam or scan line. The receiver is dynamically focused at a succession of ranges along the scan line as the reflected ultrasonic waves are received.
An ultrasound image is composed of multiple image scan lines. A single scan line (or small localized group of scan lines) is acquired by transmitting focused ultrasound energy at a point in the region of interest, and then receiving the reflected energy over time. The focused transmit energy is referred to as a transmit beam. During the time after transmit, one or more receive beamformers coherently sum the energy received by each channel, with dynamically changing phase rotation or delays, to produce peak sensitivity along the desired scan lines at ranges proportional to the elapsed time. The resulting focused sensitivity pattern is referred to as a receive beam. A scan line's resolution is a result of the directivity of the associated transmit and receive beam pair.
Scan lines are defined by their position and angle. The intersection of a beam with the transducer face is referred to as the phase center. The angle of a scan line relative to orthogonal is referred to as the steering angle.
Beamforming delays may be fixed or dynamic. Transmit delays are fixed to provide peak pressure at a particular range. Receive delays are typically dynamic since the peak sensitivity must track the increasing range r of reflections as a function of elapsed time t: ##EQU1## where c is the speed of sound in the imaged media. The elapsed time may be quantized by an amount .tau., which is equivalent to quantized focal ranges: ##EQU2## The geometry used herein is shown in FIGS. 6A and 6B for linear/sector and curved linear transducers, respectively. The important reference points are the phase center, focal point and element position. The phase center will always be the origin of the (x,z) Cartesian coordinate system. The focal point is r and the element position is p.sub.i. For curved arrays the element position is determined by the radius of curvature .rho. and the channel angle .PHI..sub.i =l.sub.i .rho., where l.sub.i is the distance from phase center along the face of the probe.
The beamformer must compensate for channel to channel differences in the propagation time T.sub.p of sound traveling between phase center and p.sub.i via a reflector at r. The relative delay T.sub.d is the difference between the propagation time for channel i and the propagation time for the phase center. For the geometry in FIG. 6A, the times T.sub.p and T.sub.d are as follows: ##EQU3##
Referring to FIG. 1, the ultrasonic imaging system incorporating the invention includes a transducer array 10 comprised of a plurality of separately driven transducer elements 12, each of which produces a burst of ultrasonic energy when energized by a pulsed waveform produced by a transmitter 22. The ultrasonic energy reflected back to transducer array 10 from the object under study is converted to an electrical signal by each receiving transducer element 12 and applied separately to a receiver 24 through a set of transmit/receive (T/R) switches 26. The T/R switches 26 are typically diodes which protect the receive electronics from the high voltages generated by the transmit electronics. The transmit signal causes the diodes to shut off or limit the signal to the receiver. Transmitter 22 and receiver 24 are operated under control of a scan controller 28 responsive to commands by a human operator. A complete scan is performed by acquiring a series of echoes in which transmitter 22 is gated ON momentarily to energize each transducer element 12, and the subsequent echo signals produced by each transducer element 12 are applied to receiver 24. A channel may begin reception while another channel is still transmitting. The receiver 24 combines the separate echo signals from each transducer element to produce a single echo signal which is used to produce a line in an image on a display monitor 30.
Transmitter 22 drives transducer array 10 such that the ultrasonic energy produced is directed, or steered, in a beam. To accomplish this, transmitter 22 imparts a time delay to the respective pulsed waveforms W. that are applied to successive transducer elements 12 via respective beamformer channels. Each channel has a respective pulser associated therewith. By adjusting the pulse time delays appropriately in a conventional manner, the ultrasonic beam can be directed away from axis 36 by an angle .theta. and/or focused at a fixed range R. A sector scan is performed by progressively changing the time delays in successive excitations. The angle .theta. is thus changed in increments to steer the transmitted beam in a succession of directions.
The echo signals produced by each burst of ultrasonic energy reflect from objects located at successive ranges along the ultrasonic beam. The echo signals are sensed separately by each transducer element 12 and the magnitude of the echo signal at a particular point in time represents the amount of reflection occurring at a specific range. Due to the differences in the propagation paths between a reflecting point P and each transducer element 12, however, these echo signals will not be detected simultaneously and their amplitudes will not be equal. Receiver 24 amplifies the separate echo signals, imparts the proper time delay to each, and sums them to provide a single echo signal which accurately indicates the total ultrasonic energy reflected from point P located at range R along the ultrasonic beam oriented at the angle .theta..
To simultaneously sum the electrical signals produced by the echoes impinging on each transducer element 12, time delays are introduced into each separate beamformer channels of receiver 24. The beam time delays for reception are the same delays as the transmission delays described above. However, the time delay of each receiver channel is continuously changing during reception of the echo to provide dynamic focusing of the received beam at the range R from which the echo signal emanates.
Under the direction of scan controller 28, receiver 24 provides delays during the scan such that steering of receiver 24 tracks the direction .theta. of the beam steered by transmitter 22 and provides the proper delays and phase shifts to dynamically focus at points P along the beam. Thus, each transmission of an ultrasonic pulse waveform results in the acquisition of a signal with a magnitude which represents the amount of reflected sound from anatomy located along the ultrasonic beam.
A detector 25 converts the received signal to display data. In the B-mode (greyscale), this would be the envelope of the signal with some additional processing such as edge enhancement and logarithmic compression.
Scan converter/interpolator 32 receives the display data from detector 25 and converts the data into the desired image for display. In particular, the scan converter converts the acoustic image data from polar coordinate (R-.theta.) sector format or Cartesian coordinate linear array to appropriately scaled Cartesian coordinate display pixel data at the video rate. This scan-converted acoustic data is then output for display on display monitor 30, which images the time-varying amplitude of the envelope of the signal as a grey scale.
Referring to FIG. 2, the receiver comprises a receive beamforming section 34 and a signal processor 38. The receive beamforming section 34 of receiver 24 includes separate beamformer channels 35. Each beamformer channel 35 receives the analog echo signal from a respective transducer element. The beamformer controller 50 converts scan line and transmit focus numbers to addresses into a channel control memory 54 (see FIG. 4). The scan controller 28 (FIG. 1) and beamformer controller 50 (FIG. 2) are loaded by the system host CPU in response to user actions such as changing the display format or connecting a different ultrasound probe.
As seen in FIG. 3, each beamformer channel 35 comprises a receive channel and a transmit channel, each channel incorporating delay means 40 and 42 respectively, which are controlled to provide the needed beamforming delays by receive control logic 44 and transmit control logic 46 respectively. Transmit is typically done by using a counter to delay the start of transmit pulse generation. Some systems may also apply relative phase rotations in addition to, or in place of, delays for receive. The receive channels also have circuitry 48 for apodizing and filtering the receive pulses.
The signals entering the summer 36 (see FIG. 2) have been delayed so that when they are summed with delayed signals from each of the other beamformer channels 35, the summed signals indicate the magnitude and phase of the echo signal reflected from anatomy located along the steered beam (.theta.). Signal processor 38 receives the beam samples from the summer 36 and produces an output to scan converter 32 (see FIG. 1).
Referring to FIG. 4, most conventional designs for the receive or transmit channel control perform beam and channel-dependent complex computations on a single central processing unit 58 and store the results in a large channel control memory 54. The channel control memory 54 is loaded by the system host CPU 58 and receives addresses corresponding to the scan line or focus number from the beamformer controller 50. Channel control of beamforming delays is typically provided by some type of delay generator logic 56 which receives control parameters from control memory 54. The control memory must contain all the necessary control parameters associated with that channel for each beam. The total amount of memory required for a 128-channel beamformer, to produce 1024 beams, is 128.times.1024.times.N, where N is the number of control parameters. These control parameters are then transmitted to the receive control logic or the transmit control logic as needed.